Field of the Invention
The present invention relates to a processing method of improving long-term transmission loss characteristics (hydrogen characteristics) due to Non-Bridging Oxygen Hole Centers (NBOHC) in an optical fiber after drawing a silica-based optical fiber. Particularly, the present invention relates to a method of determining the temperature at which an optical fiber is left in air in the processing method and an estimation method of optimally estimating the amount of an oxygen bridging element (deuterium or hydrogen) necessary for processing NBOHC in the processing method.
Description of the Related Art
Generally, in a step of drawing an optical fiber using a silica-based glass, a glass preform is subjected to a high temperature, is extended at a large tension, and furthermore is rapidly cooled. In the foregoing step, it is known that when an optical fiber is quickly cooled in a state where a glass structure is broken, silica bonding (Si—O—Si) is broken at the cooling point, and the point (Si—O.) at which non-bridging oxygen exists may occur as a defect at the cooling point. The foregoing defect is commonly referred to as a non-bridging oxygen defect or a non-bridging oxygen hole center (NBOHC: Non Bridging Oxygen Hole Center). In the case where an optical fiber containing a large amount of the above-described NBOHC is used in an atmosphere including hydrogen for a long time, hydrogen (H) that is doped into the optical fiber from the atmosphere is bonded to NBOHC, a —OH group is generated, and a transmission loss thereof gradually increases near a wavelength of 1380 nm. This means that, long-term loss characteristics (hydrogen characteristics) of the optical fiber are deteriorated in an atmosphere including hydrogen.
Conventionally, as a result of carrying out deuterium treatment to an optical fiber after a silica-based optical fiber is drawn and before deterioration in a transmission loss due to hydrogen occurs, it is known that it is possible to prevent an increase in transmission loss near a wavelength of 1380 nm.
The deuterium treatment is a process of causing a drawn optical fiber to be subjected to an atmosphere including deuterium (D) that is an isotope of hydrogen, causing the deuterium to infiltrate into the optical fiber, causing the deuterium to be bonded to non-bridging oxygen (O.) of NBOHC (i.e., non-bridging oxygen is bridged by deuterium, an OD group occurs), and thereby extinguishing NBOHC. As stated above, as a result of extinguishing NBOHC in the optical fiber in advance before using the optical fiber as a product, it is possible to prevent transmission loss characteristics from being degraded due to hydrogen (hydrogen characteristics can be improved) even where hydrogen is infiltrated into the optical fiber product from the atmosphere including hydrogen in the case of using the optical fiber product after extinguishing treatment.
Methods of using the above-described deuterium treatment are specifically disclosed in, for example, Japanese Patent No. 4948718 (hereinafter, referred to as Patent Document 1) and Japanese Unexamined Patent Application, First Publication No. 2012-193102 (hereinafter, referred to as Patent Document 2).
Patent Document 1 is known as a fundamental patent regarding deuterium treatment. In the disclosure of Patent Document 1, a method including: a step of allowing an optical fiber to come into contact with a mixed gas including deuterium; and a step of subsequently removing the mixed gas in air or nitrogen is disclosed. In this method, the step of allowing the optical fiber to come into contact with the mixed gas including deuterium is carried out in the temperature range of 20° C. to 40° C.
However, Patent Document 1 does not completely disclose the reason that the optical fiber is subjected to the mixed gas including deuterium in the temperature range of 20° C. to 40° C. Therefore, it is unclear whether or not the temperature range of 20° C. to 40° C. is effective to carry out deuterium treatment in the optimal time period, and particularly it is unclear whether or not this temperature range is effective to carry out deuterium treatment in the necessary, sufficient, and shortest time period.
Furthermore, in Patent Document 1, after the step of causing the optical fiber to be subjected to the mixed gas including deuterium, the step of removing the mixed gas in air or nitrogen is carried out. Particularly, paragraph 0036 of Patent Document 1 describes, “the fiber is maintained in the mixed gas including deuterium at a constant time period in which deuterium existing in the mixed gas is doped into and sufficiently reacted with the fiber. This period is particularly determined by the temperature and factors associated with the contained amount of deuterium in the mixed gas.”. Furthermore, paragraph 0037 of Patent Document 1 describes “after completion of reaction and before shipment, in order for degasification, the optical fiber is removed and maintained in a neutral atmosphere such as nitrogen or air. During this step, excessive deuterium that similarly provides an absorption band exists in the entire fiber is discharged”.
According to the description of Patent Document 1, it is possible to read that, from the step of causing deuterium to infiltrate into the optical fiber to the step of continuously diffusing deuterium into the optical fiber, that is, until completion of deuterium treatment, an optical fiber is subjected to the mixed gas including deuterium.
In the case of continuously allowing the optical fiber to be exposed to the atmosphere including deuterium until deuterium treatment completion as described above, a processing speed increases; however, deuterium is wastefully added into the optical fiber. Moreover, not only this, but also it is known that if deuterium molecules excessively remain in the optical fiber, an excessive loss in transmission band occurs due to the effect of the excessive deuterium. For this reason, in the method disclosed in Patent Document 1, the step of removing the deuterium gas is carried out after completion of deuterium treatment (infiltration and diffusion of deuterium). However, in this method, since a long period of time is required for degasification, as a result, it is conceivable that the total processing time cannot be shortened.
Because of this, it is thought that, deuterium is infiltrated into the optical fiber by exposing the optical fiber to the atmosphere including deuterium in the minimum time required for exposure so that deuterium is not excessively added to the optical fiber, thereafter the degasification time is shorted by diffusing the deuterium into the inside of the optical fiber in air. In this case, the rate of diffusion is lower than that in the case of continuously exposing the optical fiber to the atmosphere including deuterium; however, in terms of results, it is believed that the total length of time decreases.
In the technique disclosed in Patent Document 2, it is believed that the method described above is utilized which allows deuterium to be infiltrated into the optical fiber by exposing the optical fiber to the atmosphere including deuterium in the minimum time required for exposure, and thereafter diffuses the deuterium into the inside of the optical fiber in air.
A method of processing an optical fiber disclosed in Patent Document 2 basically includes: a diffusion step of exposing a silica glass to an atmosphere including deuterium in a predetermined amount of time and thereby diffusing deuterium molecules into the silica glass; a high-temperature maintaining step of maintaining the silica glass at a temperature of 40° C. or higher; and a cooling step of subsequently cooling the silica glass so that the temperature thereof becomes a room temperature as described in claim 1 thereof.
Specifically, regarding the processing method disclosed in Patent Document 2, as described in process 1 of (Optical Fiber Deuterium Treatment) in paragraphs 0023 to 0026, steps are disclosed in which “deuterium exposure at a room temperature for approximately 1 day”, “air atmosphere exposure for approximately 1 day”, and “a step of maintaining at high-temperature in an air atmosphere at 40° C.” are carried out in this order. According to the description of Patent Document 2, it is possible to read that, “deuterium exposure at a room temperature for approximately 1 day” and “air atmosphere exposure for approximately 1 day” correspond to “a diffusion step” described in claim 1. That is, it is conceivable that “diffusion step” described in claim 1 of Patent Document 2 includes the steps in which the optical fiber is exposed to the deuterium atmosphere, deuterium is infiltrated into the optical fiber, the optical fiber is further subjected to an air atmosphere, and deuterium is diffused into the optical fiber. Particularly, in the method disclosed in Patent Document 2, it is believed that, the reason why a high temperature is maintained by making a temperature higher after completion of the diffusion step is because it is necessary to expedite degasification by raising the temperature since a large amount of deuterium is doped into the optical fiber. Moreover, Patent Document 2 also does not disclose that the temperature in the air atmosphere exposure (air atmosphere exposure before the step of maintaining at high-temperature at 40° C.) shortly after deuterium exposure at a room temperature is adjustable in accordance with the optical fiber serving as the processing target.
Additionally, as a process of improving the hydrogen characteristics, in order to prevent an increase in transmission loss due to NBOHC near a wavelength of 1380 nm in a commonly-used optical fiber for communication from increasing, a processing using a gas including deuterium is generally carried out. Furthermore, as described below again, special optical fibers (optical fibers or the like used in a wavelength region in which a transmission loss near a wavelength of 1380 nm does not particularly interfere therewith) may be processed by using a gas including hydrogen. Hydrogen bonds to NBOHC, thereby generates an OH group (Consequently, NBOHC is extinguished), and make a transmission loss near a wavelength of 1380 nm higher. Regarding special optical fibers which use, as a transmission band, a wavelength region in which a transmission loss near a wavelength of 1380 nm does not particularly interfere therewith, it is conceivable that hydrogen processing is carried out in advance instead of a deuterium treatment. In the present description, both deuterium and hydrogen are referred to as an oxygen bridging element that bonds (bridges) to non-bridging oxygen in the optical fiber and can process non-bridging oxygen holes. However, in the below description, as a representative example of the oxygen bridging element, the oxygen bridging element may simply be referred to as deuterium.
As evidenced by each of the aforementioned patent documents, infiltration of deuterium into the optical fiber and diffusion of deuterium into the optical fiber require a long amount of time. Accordingly, as long as after a long period of time elapses and after the start of deuterium treatment, it cannot be determined whether or not a practically-sufficient amount of deuterium can be included in the optical fiber, that is, whether or not the effect due to the deuterium treatment is sufficiently obtained. The specific time varies depending on the conditions of producing the optical fiber or the conditions of the deuterium treatment; however, a degree of the effect due to the deuterium treatment can be measured after 1 to 2 days or more elapse at the earliest. Accordingly, in this case, unless 1 to 2 days or more elapse, it is not possible to evaluate the effect due to deuterium treatment.
For this reason, conventionally, a processing time is generally set to be longer than necessary so as to reliably evaluate the effect due to deuterium treatment. However, in the case of unnecessarily setting the processing time to be longer, deuterium excessively remains in the optical fiber as mentioned above. Because of this, in order to prevent loss characteristics from being deteriorated due to the excessive deuterium, it is necessary to add a degasification step described in Patent Document 1 to treatment steps. As a result, the total processing time becomes further longer.
In contrast, in the case of simply raising the temperature in the diffusion step in order to speed up the throughput speed, the diffusion of deuterium in the direction toward the center of the optical fiber is expedited, and there is a possibility that the processing time can be shortened; however, in the case of unnecessarily raising the temperature in this step, the diffusion in the direction toward the outside of the optical fiber is expedited. Consequently, even in the case of raising the temperature, unless the temperature is set to a suitable temperature, diffusion and removal of deuterium toward the outside of the optical fiber is quickly carried out, and the effect due to the deuterium treatment cannot be sufficiently obtained. However, in conventional technique, it is not completely considered that a processing time is shortened as a result of suitably adjusting the temperature in consideration of the above-described diffusion of deuterium toward the outside of the optical fiber.